SP4427DS/01

SP4427 Electroluminescent Lamp Driver

© Copyright 1997 Sipex Corporation

4

voltage in the system (up to the maximum tolerance

of the coil) and the coil should have low resistance

and high inductance; V

20mH/70

Ω coil are typical. The majority of the

current goes through the coil and typically less

than 1mA is required for Vdd of the SP4427. Vdd

can range from 1.1V to 5V; it is not necessary that

Vdd=V

material and winding used -- performance variances

may be noticeable from different coil suppliers.

The Sipex SP4427 is final tested using a 20mH/

70

Ω coil from CTC. For suggested coil sources see

page 7.

The F

end of the coil at pin 3 to ground or to open circuit.

The F

the time when the F

connected from V

magnetic field is created in the coil. During the low

part of F

open, the field collapses and the energy in the

inductor is forced to flow toward the

high voltage H-bridge switches. F

coil

will send

as many charge pulses as possible in 1 Lamp Cycle

{Number of Coil pulses in 1 lamp cycle =

Coil Freq

}(see figure 2 page 6). Each pulse

increases the voltage drop across the lamp

in discrete steps. As the voltage potential

approaches its maximum, the steps become smaller

(see figure 1 page 6).

The H-bridge consists of two SCR structures that

act as high voltage switches. These two switches

control the polarity of how the lamp is charged.

The SCR switches are controlled by the F

which is the oscillator frequency divided by 256.

For a 64kHz oscillator, F

0.1

µF

LAMP

1

Cosc

Vss

Coil

EL2

Losc

Hon

Vdd

EL1

+

-

1.5 Volt

Application

3.FtL

1.6 SQ. IN.

Typical SP4427CN Application Circuit

When the energy from the coil is released, a high

voltage spike is created triggering the SCR

switches. The direction of current flow is

determined by which SCR is enabled. One full

cycle of the H-bridge will create a number of

voltage steps from ground to 55V (typ) on pins 4

and 5 which are 180 degrees out of phase with each

other (see figure 3 page 6). A differential view of

the outputs is shown in figure 4 on page 6. If Line

Noise is of concern it is advisable to add a

decoupling cap at Vdd (low ESR type).

Electroluminescent Technology

What is electroluminescence?

An EL lamp is basically a strip of plastic that is

coated with a phosphorous material which emits

light (fluoresces) when a high voltage (>40V)

which was first applied across it, is removed or

reversed. Long periods of DC voltages applied to

the material tend to breakdown the material and

reduce its lifetime. With these considerations in

mind, the ideal signal to drive an EL lamp is a high

voltage sine wave. Traditional approaches to

achieving this type of waveform included discrete

circuits incorporating a transformer, transistors,

and several resistors and capacitors. This approach

is large and bulky, and cannot be implemented in

most hand held equipment. Sipex now offers low

power single chip driver circuits specifically

designed to drive small to medium sized

electroluminescent panels. All that is required is

one external inductor and capacitor.

Electroluminescent backlighting is ideal when used

with LCD displays, keypads, or other backlit

readouts. Its main use is to illuminate displays in

Lamp Freq (2)